1. Field of the Invention
The economical transportation of coal becomes increasingly important as coal use as a fuel and source of chemicals expands. It is well established that in order to have successful economic pipeline transportation of coal, a particular particle size distribution is required. The coal must have a substantial weight percentage of superfine coal particles in order to have what is referred to as a low energy slurry.
The need for having superfine particles present in the slurry has required a number of expensive process steps in the normal preparation and processing of coal for pipelining and use in chemical processing or as a fuel. Conventionally, the run of mine coal has been screened to divide the coal into a coarse coal fraction and a fine coal fraction, and the two fractions each separately cleaned and then dewatered from the water used in the cleaning. The two fractions are then recombined and ground to provide the desired proportion of superfine particles for pipelining. After pipelining, the coal must be retained as a slurry because of the presence of the superfine particles. If the coal is dewatered and stockpiled at this stage, the superfines can cause spontaneous combustion and are subject to being wind blown, which can result in a thin layer of coal fines spread over a wide area, resulting in pollution and coal losses. When the coal in the slurry or slurry pond is required for use, it is dewatered and because of the presence of the superfines, expensive centrifugation equipment and slurry heating are required to remove the water.
There is substantial interest in devising new techniques which will allow for more economical and efficient preparation of coal for pipelining and use of the coal for subsequent processing, particularly for furnaces, coal liquefaction and the like. Desirably, new processing should minimize expensive grinding and dewatering, and maximize the efficiency of coal transportation in the pipeline.
While coal has assumed increasing importance, the economic transporation of other ores also remains of considerable interest.
2. Description of the Prior Art
U.S. Pat. Nos. 2,920,923, 3,168,350, 3,637,263 and 3,719,397 all describe various techniques for pipelining coal. The last patent, U.S. Pat No. 3,719,397, describes a process involving an insoluble solid carrier for use as superfines in pipelining coal. The process initially divides the coal into two fractions, a coarse and fine fraction, adds the superfine insoluble carrier to the coarse fraction, and then pipelines the two fractions separately. The coarse fraction is then screened to remove the insoluble solid carrier superfines and then superfines returned to the coal mine site for reuse with the coarse coal fraction.
The fine coal fraction is ground to the size consistency or distribution necessary for coal pipelining and after pipelining dewatered at the user end of the pipeline. This dewatering is the same as that presently used commercially in coal pipelining because the fine coal being pipelined contains superfine fractions, as the fine coal is ground before pipelining to make the superfines.
U.S. Pat. No. 3,719,397 thus does not change the fine fraction pipelining conditions, as the fine fraction defined therein represents about 50% of the run of mine coal. To utilize the above patent invention requires "slug" operation, which not only adds the inconvenience of intermediate pipeline flushing, but decreases coal capacity of the pipeline. Further, the above patent invention requires additional costs for grinding of the coal and dewatering.